Identification, friend or foe (IFF) is an identification system designed for command and control. It enables military and national (civilian air traffic control) interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator. IFF may be used by both military and civilian aircraft.

Despite the name, IFF can only positively identify friendly targets, not hostile ones.[1][2][3][4] If an IFF interrogation receives no reply or an invalid reply, the object cannot be identified as friendly, but is not positively identified as foe. There are in addition many reasons that friendly aircraft may not properly reply to IFF.

IFF is a tool within the broader military action of Combat Identification (CID), "the process of attaining an accurate characterization of detected objects in the operational environment sufficient to support an engagement decision." The broadest characterization is that of friend, enemy, neutral, or unknown. CID not only can reduce friendly fire incidents, but also contributes to overall tactical decision-making.[5]

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With the successful deployment of radar systems for air defence during World War II, combatants were immediately confronted with the difficulty of distinguishing friendly aircraft from hostile ones; by that time, aircraft were flown at high speed and altitude, making visual identification impossible, and the targets showed up as featureless blips on the radar screen. This led to incidents such as the "battle of Barking Creek", over Britain,[6][7][8] and the "air attack on the fortress of Koepenick", over Germany.[9][10]

Already before the deployment of their Chain Home radar system (CH), the RAF had considered the problem of IFF. Robert Watson-Watt had filed patents on such systems in 1935 and 1936 respectively. By 1938, researchers at Bawdsey Manor began experiments with "reflectors" consisting of a dipole antennas tuned to resonate at the primary frequency of the CH radars. When a pulse from the CH transmitter hit the aircraft, the antennas would resonate for a short time, increasing the amount of energy returned to the CH receiver. The antenna was connected to a motorized switch that periodically shorted it out, preventing it from producing a signal. This caused the return on the CH set to periodically lengthen and shorten as the antenna was turned on and off. In practice, the system was found to be too unreliable to use; the return was highly dependent on the direction the aircraft was moving relative to the CH station, and often returned little or no additional signal.[11]

It was suspected this system would be of little use in practice, and when that turned out to be the case the RAF turned to an entirely different system that was also being planned. This consisted of a set of tracking stations using HF/DFradio direction finders. Their aircraft radios were modified to send out a 1 kHz tone for 14 seconds every minute, allowing the tracking stations ample time to measure the aircraft's bearing. Several such stations were assigned to each "sector" of the air defence system, and sent their measurements to a plotting station at sector headquarters, who used triangulation to determine the aircraft's location. Known as "pip-squeak",[12] the system worked but was labour-intensive and did not display its information directly to the radar operators. A system that worked directly with the radar was clearly desirable.

The first active IFF transponder (transmitter/responder) was the 'IFF Mark I' and was put into operation in 1939. On receipt of a signal from the CH radar (20-30 MHz), an oscillator in the system began to ring with the same frequency. This signal was amplified and sent out an omnidirectional monopole antenna, where it was received by the CH station. The oscillator circuit rang only for a short time, causing the signal to quickly disappear again. Since the signal was received at the same time as the original reflection of the CH signal, the result was a distorted "blip" on the CH display which was easily identifiable. Mark I was technically complete as the war began, but a lack of sets meant it was not available in quantity and only a small number of RAF aircraft carried it by the time of the Battle of Britain. Pip-squeak was kept in operation during this period, but as the Battle ended, IFF Mark I was quickly put into operation. Pip-squeak was still used for areas over land where CH did not cover, as well as an emergency guidance system.[13][better source needed]

The primary problem with the Mark I was that it operated at a single set frequency. If the aircraft moved between CH stations, it had to be manually re-tuned, based on settings read off a card. As the number of radars in British service increased, including new systems deployed by the navy and army, an aircraft might find itself painted by several radars at the same time. This led to the introduction of IFF Mark II, whose primary change was a circuit that automatically changed the tuned frequency, sweeping it through the bands being used by the various radars. Each radar would see the IFF returns appear for a short period every few seconds. Even this solution became untenable with the introduction of microwave frequency radars, which dramatically increased the range of frequencies in use.[13][better source needed]

In 1940, English engineer Freddie Williams began work on the Mark III system at the Telecommunications Research Establishment, which was to become the standard for the Western Allies for most of the war. Mark III transponders were designed to respond to specific 'interrogators', rather than replying directly to received radar signals. These interrogators worked on a limited selection of frequencies, no matter what radar they were paired with. The system also allowed limited communication to be made, including the ability to transmit a coded 'Mayday' response. The IFF sets were designed and built by Ferranti in Manchester to Williams' specifications. Equivalent sets were manufactured in the US, initially as copies of British sets, so that allied aircraft would be identified upon interrogation by each other's radar.[13][better source needed]

FuG 25a "Erstling" (English: "Firstborn", "Debut") was developed in Germany in 1940. It received the radar frequencies on the low-VHF band at 125 MHz used by the Freya radar and the low-UHF-banded 550–580 MHz used by Würzburg).

Before flight, the transceiver was set up with a selected day code of ten bits which was dialed into the unit. To start the identification procedure, the ground operator switched the pulse frequency of his radar from 3,750 Hz to 5,000 Hz. The airborne receiver decoded that and started to transmit the day code. The radar operator would then see the blip lengthen and shorten in the given code, ensuring it was not being spoofed. The IFF transmitter worked on 168 MHz with a power of 400 watts (PEP).

British military scientists found a way of exploiting this system to the allies' advantage. They designed and built their own IFF transmitter called "Perfectos", which was designed to trigger a response from any FuG 25a system in the vicinity. When mounted in an RAFMosquito, the "Perfectos" device revealed the position of any German nightfighters fitted with an FuG 25a. As a result, the British "Perfectos" device severely compromised German use of the FuG 25a.

The IFF of World War II and Soviet military systems (1946 to 1991) used coded radar signals (called Cross-Band Interrogation, or CBI) to automatically trigger the aircraft's transponder in an aircraft illuminated by the radar. Radar-based aircraft identification is also called secondary radar in both military and civil usage, with primary radar bouncing an RF pulse off of the aircraft to determine position. George Charrier, working for RCA, filed for a patent for such an IFF device in 1941. It required the operator to perform several adjustments to the radar receiver to suppress the image of the natural echo on the radar receiver, so that visual examination of the IFF signal would be possible.[14]

By 1943, Donald Barchok filed a patent for a radar system using the acronym IFF in his text with only parenthetic explanation, indicating that this acronym had become an accepted term.[15] In 1945, Emile Labin and Edwin Turner filed patents for radar IFF systems where the outgoing radar signal and the transponder's reply signal could each be independently programmed with a binary codes by setting arrays of toggle switches; this allowed the IFF code to be varied from day to day or even hour to hour.[16][17]

The United States and other NATO countries started using a system called Mark XII in the late twentieth century; Britain had not until then implemented an IFF system compatible with that standard, but then developed a program for a compatible system known as successor IFF (SIFF).[18]